FIG. 1 shows a typical optical lithographic fabrication system 100 for delineating features in a workpiece 120. Typically the workpiece comprises a semiconductor wafer (substrate), together with one or more layers of material(s) (not shown) located on a top major surface of the water. More specially, monochromatic optical radiation of wavelength .lambda. emitted by an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical collimating lens 104, a lithographic mask of reticle 103, and an optical focusing lens 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the workpiece 120. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a development process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the workpiece 120. Portions of the workpiece 120 thus are removed from the top surface of the workpiece 120 at areas underlying those where the photoresist layer 101 was removed by the development process but not at areas underlying those regions where the photoresist remains. Alternatively, instead of etching the workpiece, impurity ions can be implanted into the workpiece 120 at areas underlying those where the photoresist layer was removed by the development process but not at areas underlying where the photoresist remains. Thus, the pattern of the mask 103--i.e., each feature of the mask--is transferred to the workpiece 120 as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and other. Thus, for example, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, a feature having a width equal to C must be located on the mask (reticle) 103. According to geometric optics, if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become indistinct; hence the resolution of the features of the reticle 103, as focused on the photoresist layer and transferred to the workpiece, deteriorates.
In order to improve the resolution, in the prior art phase-shifting lithographic masks have been taught. A phase-shifting mask is used as the mask (or reticle) 103 in the system 100. Moreover, a phase-shifting mask comprises various regions that impart various phase shifts to the optical radiation that originates from the source 106 and propagates through the mask 103 in the system 103. More specifically, these phase-shifting masks have opaque and transparent regions. The transparent regions typically have at least two different thicknesses suitable for imparting at least two different phase shifts, typically 0 and .pi. radians, to the optical radiation (of wavelength .lambda.) propagating through the mask when it is being used as the reticle 103.
For example, as shown in FIG. 2, a phase-shifting mask 200, typically a slab of (optically transparent) quartz, comprises an optically opaque region 201 located at a top surface of the mask. This opaque region 201 is bounded by a pair of edges running parallel to the Y-direction and terminating at say Y=Y.sub.0.
The opaque region 201 typically can be supplied by an opaque metallic region. Alternatively, the opaque region 201, it can be supplied by a diffraction grating region that is constructed so as to diffract light away from the zero'th order of the resulting diffraction pattern, and thus the diffraction grating region is equivalent to an opaque region when the mask 200 is used as the reticle 103 in the system 100. Such a diffraction grating region, however, requires a higher resolution than does the remainder (transparent regions) of the mask, whereby undesirably long fabrication times, as well as undesirably stringent geometric tolerances, are required.
The opaque region 201 separates portions of optically transparent regions 210 and 211, which respectively impart phase shifts .PHI..sub.0 and .PHI..sub.1, typically approximately equal to 0 and .pi. radian, respectively. On the other hand, a transparent transition region 212, which is also located between regions 210 and 211, separates other portions of these regions 210 and 211 from each other. This transition region 212 imparts a phase shift that is intermediate .PHI..sub.0 and .PHI..sub.1, typically approximately .pi./2 radian, that of itself does not produce a separate (third) region in the image on the photoresist layer 101 but produces only a transition region in the mask 200. The transition region 212 can also be subdivided, as indicated in phase-shift mask 300 (FIG. 3), into transparent subregions 221 and 222 that impart phase shifts equal to approximately .pi./3 and 2.pi./3 radian, respectively.
When using the mask 200 or 300 as the reticle 103 in the system 100, the optical radiation from the source 106 will be focused as an image on the photoresist layer 102 in such a manner that this image will have a feature running parallel to the Y-direction and terminating along an edge parallel to the X-direction. Thus, after the development process the photoresist layer will be patterned; more specifically, it will have a feature bounded by a pair of parallel edges running parallel to the Y-direction and terminating at say Y=Y.sub.1, (not shown) where Y=Y.sub.1 is the image of Y=Y.sub.0. Finally, the thus patterned photoresist layer can be used to transfer its pattern to the underlying workpiece 120, as by means of ion implantation, or by means of dry or wet etching.
In a paper authored by H. Ohtsuka, et al. entitled "Conjugate Twin-Shifter for the New Phase Shift Method to High Resolution Lithography," published by SPIE (The Society of Photo-Optical Instrumentation Engineers), The International Society for Optical Engineers, in the Proceedings of Optical/Laser Microlithography IV, Vol. 1463, pp. 112-123, (Mar. 6-8, 1991), multiple phase-shifting transparent regions apparently were formed by successive anisotropic etching (successive vertical cuts) to mutually differing depths in a phase-shifting layer. Each of these etchings, however, require a separate alignment step for aligning the boundary edges of the phase-shifting region to be etched with the boundary of edges of a phase-shifting region that has already been etched.
This requirement of separate (multiple) alignment steps has one or more of the following disadvantages. First, formation of alignment marks is required during the first alignment, and these marks must be plated with metal (to remove accumulated electrical charge) so that they can be detected--i.e., their locations can be ascertained--during each of the subsequent alignment steps. This formation, plating, and repeated detection of the alignment marks entails an undesirably large amount of costly effort. Second, each of the etchings requires a separate spinning-on and patterning of a fresh resist layer. Each of the required resist layers undesirably adds to costs of (resist) materials. Third, removal of each resist layer produces particulate matter so that a careful cleaning step is required prior to spinning-on the next resist layer to avoid defects in the mask otherwise caused by this particulate matter. Fourth, development of each resist layer likewise produces particulate matter that locally spoils the etching properties, so that a careful cleaning step is required between each development and etching of a given resist layer. Since any added cleaning steps take time, these cleaning steps undesirably add to processing time and costs. Fifth, any alignment with respect to alignment marks is not as precise as an alignment of a single beam of actinic radiation--that is, a beam of radiation that modifies the development properties of a resist, such as an electron beam, a photon beam, or an ion beam. Although the effects of such misalignment can be avoided by restricting some, if not all, discontinuities in thickness of the transparent material to be located at positions overlying opaque (metallic) material, the kinds of geometries of images that can be handled by the phase-shifting mask when used as the reticle 103 are undesirably limited.
In a paper authored by H. Watanabe et al. entitled "Transparent phase shifting mask with multistage phase shifter and comb-shaped shifter," published by SPIE in the above-mentioned Proceedings at pp. 101-110, multiple phase shifts in a phase-shifting masks were obtained by using the multiple phase shifts produced in a resist layer that had multiple thicknesses. The resist layer was located on a transparent parallel slab. The multiple thicknesses were produced by development of a resist layer, initially of uniform thickness prior to such development, having four regions that had been implanted with four mutually different dose levels (i.e., charge per unit area, one of them equal to zero) of electrons. A disadvantage of this method is that the resist layer is an essential phase-shifting element that remains in the phase-shifting mask. Such a resist layer cannot be repaired easily in case it has defects, is not stable with the passage of time (during subsequent use as the reticle 103 in the system 100), and introduces undesirable optical reflections owing a resulting discontinuity of refractive index at the interface of the resist layer and the underlying transparent slab.
Therefore, it would be desirable to have a method of forming a phase-shifting mask having three or more phase-shifting regions which impart three or more mutually differing respective phase shifts to the optical radiation of wavelength .lambda. propagating through mask, but which does not require a separate alignment step between each etching and which does not rely on multiple phase shifting by resist material.
In addition it would be further desirable to have a single-alignment method of forming just two phase-shifting regions--such as regions 210 and 211--that impart two different phase shifts and that are separated by an opaque region, as exemplified by the cut 11--11 shown in FIG. 2. And it would be yet further desirable to have a self-aligned method of forming an opaque region that does not require a diffraction grating.